Operation Manual

Experiments in Python
99
Notes:
That “action” is stored in a computer as numbers ranging from some large
negative number to the same positive number, say 16,384 (2
14
– remember your
binary!), for a particular duration of time, say 4 seconds. If we move up and down
that range evenly, say, 440 times each second, and pass those numbers to the
sound hardware in the Raspberry Pi, we should hear the musical note “A”, above
middle C.
The evenness of this movement is given by the mathematic function we saw
earlier, called sine, which when given an angle ranges from -1 to +1 for every
180 degrees. Given the full 360 degrees it goes from -1 to +1 and then back to -1
again. This gives the peaks and troughs. The “sin” function in Python doesn’t use
degrees, it uses “radians”. There are 2π radians in 360 degrees.
We can’t just pass 440 peaks and troughs every second because we’d miss out
all those values in-between. In the digital music world, CDs and MP3s send out
44,100 numbers to get the numbers in-between each second of sound. We can do
that too, and this gives us the sampling rate of the music.
Holding a lot of data in Python can be inefficient, so some clever solutions for
holding numbers can be found in a library called “numpy”. The first “arange”
generates a long list of consecutive numbers – 44,100 for each second of music.
Each is a point in time of each sample of sound. The “sin” function takes the list
of numbers, adjusted by omega for the correct frequency, and gives a list of
numbers representing the movement of the air for each point in time.
This list of sounds can then be saved to a file in a particular wave format. Finally,
that sound wave can be played in the sound mixer in PyGame.
Normally, you wouldn’t generate sounds and then play them immediately. Sound
files such as these would be created in advance, and only played within your own
programs when required. However, if you were developing a sound-manipulation
program, you would need to create and manipulate the sound data directly, a little
like this. There are better methods of doing this that do not require writing to a file,
which you might think about.
The sin() function returns
a numeric value between
-1 and +1, which represents
the sine of the parameter
entered between the
brackets.